import 'package:halkyon_learn_flutter/src/core/content/rich_content.dart';
import 'package:halkyon_learn_flutter/src/core/profile/public_person.dart';
import 'package:halkyon_learn_flutter/src/feature/discussion/data/discussion_gateway.dart';
import 'package:halkyon_learn_flutter/src/feature/discussion/model/discussion.dart';

/// The instance's discussions in memory, as the person [me] (the
/// studyset's author when [author]) sees them; every question on a studyset
/// titled [studysetTitle]. Records what was written.
class FakeDiscussionGateway({
  final PublicPerson me = const PublicPerson(handle: 'me'),
  final bool author = false,
  final String studysetTitle = 'Japanese',

  /// Why the person may not write; null when they may.
  var DiscussionRefusal? refusal,
}) implements DiscussionGateway {
  /// Thrown by every call while set: the instance can't be reached.
  Exception? fail;

  /// The questions, by id, with what is in them.
  final questions = <String, Discussion>{};

  /// The topics each question is within.
  final topicsOf = <String, Set<String>>{};

  /// The discussions marked read, in order.
  final seen = <String>[];

  /// The handles blocked.
  final blocked = <String>[];

  var _next = 0;

  /// Adds a question by [by] about [target], with [answers]; returns it.
  Discussion addQuestion(
    String title, {
    String publicId = 'pub-1',
    PublicPerson by = const PublicPerson(handle: 'bob'),
    DiscussionTarget target = const DiscussionTarget(
      kind: DiscussionTargetKind.studyset,
      item: 'root',
      version: 1,
    ),
    Set<String> topics = const {'root'},
    List<DiscussionPost> answers = const [],
    List<DiscussionPost> comments = const [],
    String? acceptedAnswerId,
    String text = 'Why?',
  }) {
    final id = 'q${_next++}';
    final discussion = Discussion(
      question: DiscussionPost(
        id: id,
        kind: DiscussionPostKind.question,
        createdAt: DateTime.utc(2026, 10, 15, 12),
        by: by,
        title: title,
        content: RichContent.plain(text),
        mine: by == me,
      ),
      publicId: publicId,
      studysetTitle: studysetTitle,
      location: 'halkyon.eu/@alice/japanese',
      target: target,
      answers: answers,
      comments: comments,
      acceptedAnswerId: acceptedAnswerId,
      asker: by == me,
      author: author,
      refusal: refusal,
    );
    questions[id] = discussion;
    topicsOf[id] = topics;
    return discussion;
  }

  /// An answer by [by].
  DiscussionPost answerBy(
    String text, {
    PublicPerson by = const PublicPerson(handle: 'carol'),
    String? id,
  }) => DiscussionPost(
    id: id ?? 'a${_next++}',
    kind: DiscussionPostKind.answer,
    createdAt: DateTime.utc(2026, 10, 15, 13),
    by: by,
    content: RichContent.plain(text),
    mine: by == me,
  );

  void _failing() {
    if (fail case final error?) {
      throw error;
    }
  }

  void _writing() {
    _failing();
    if (refusal case final refusal?) {
      throw DiscussionRefusedException(refusal);
    }
  }

  DiscussionSummary _summary(Discussion discussion) => DiscussionSummary(
    questionId: discussion.question.id,
    publicId: discussion.publicId,
    studysetTitle: discussion.studysetTitle,
    title: discussion.question.title,
    asker: discussion.question.by,
    askerIsAuthor: discussion.question.fromAuthor,
    target: discussion.target,
    createdAt: discussion.question.createdAt,
    activeAt: discussion.question.createdAt,
    answers: discussion.answers.length,
    accepted: discussion.acceptedAnswerId != null,
    mine: discussion.asker,
    unseen: !seen.contains(discussion.question.id),
  );

  @override
  Future<List<DiscussionSummary>> list(
    String publicId, {
    String? topic,
    DiscussionTarget? item,
    String? query,
    int page = 0,
  }) async {
    _failing();
    return [
      for (final MapEntry(key: id, value: discussion) in questions.entries)
        if (discussion.publicId == publicId &&
            (topic == null || topicsOf[id]!.contains(topic)) &&
            (item == null || discussion.target == item) &&
            (query == null ||
                discussion.question.title.toLowerCase().contains(
                  query.toLowerCase(),
                )))
          _summary(discussion),
    ].reversed.toList();
  }

  @override
  Future<Discussion> read(String questionId) async {
    _failing();
    return questions[questionId] ?? (throw DiscussionMissing());
  }

  @override
  Future<DiscussionRefusal?> askRefusal(String publicId) async {
    _failing();
    return refusal;
  }

  @override
  Future<String> ask(
    String publicId,
    DiscussionTarget target,
    String title,
    RichContent content,
  ) async {
    _writing();
    return addQuestion(
      title,
      publicId: publicId,
      by: me,
      target: target,
      topics: {target.item},
      text: content.plainText,
    ).question.id;
  }

  @override
  Future<String> answer(String questionId, RichContent content) async {
    _writing();
    final discussion = questions[questionId] ?? (throw DiscussionMissing());
    final answer = DiscussionPost(
      id: 'a${_next++}',
      kind: DiscussionPostKind.answer,
      createdAt: DateTime.utc(2026, 10, 15, 14),
      by: me,
      fromAuthor: author,
      content: content,
      mine: true,
    );
    questions[questionId] = _with(
      discussion,
      answers: [
        ...discussion.answers,
        answer,
      ],
    );
    return answer.id;
  }

  @override
  Future<String> comment(String onId, String text) async {
    _writing();
    final MapEntry(key: id, value: discussion) = questions.entries.firstWhere(
      (entry) =>
          entry.key == onId ||
          entry.value.answers.any((answer) => answer.id == onId),
      orElse: () => throw DiscussionMissing(),
    );
    final comment = DiscussionPost(
      id: 'c${_next++}',
      kind: DiscussionPostKind.comment,
      createdAt: DateTime.utc(2026, 10, 15, 15),
      parentId: onId,
      by: me,
      text: text,
      mine: true,
    );
    questions[id] = _with(
      discussion,
      comments: [
        ...discussion.comments,
        comment,
      ],
    );
    return comment.id;
  }

  @override
  Future<void> edit(String postId, RichContent content, {String? title}) async {
    _writing();
  }

  @override
  Future<void> editComment(String commentId, String text) async {
    _writing();
  }

  @override
  Future<void> delete(String postId) async {
    _failing();
    questions.remove(postId);
  }

  @override
  Future<void> accept(String questionId, String? answerId) async {
    _failing();
    final discussion = questions[questionId] ?? (throw DiscussionMissing());
    questions[questionId] = _with(
      discussion,
      acceptedAnswerId: answerId,
      unaccept: answerId == null,
      answers: [
        ...discussion.answers.where((answer) => answer.id == answerId),
        ...discussion.answers.where((answer) => answer.id != answerId),
      ],
    );
  }

  @override
  Future<void> hide(String postId, {required bool hidden}) async {
    _failing();
  }

  @override
  Future<List<DiscussionSummary>> inbox() async {
    _failing();
    return [
      for (final discussion in questions.values)
        if (discussion.asker || discussion.author) _summary(discussion),
    ];
  }

  @override
  Future<int> unseen() async =>
      (await inbox()).where((summary) => summary.unseen).length;

  @override
  Future<void> markSeen(String questionId) async {
    _failing();
    seen.add(questionId);
  }

  @override
  Future<void> block(String handle) async {
    _failing();
    blocked.add(handle);
  }

  Discussion _with(
    Discussion discussion, {
    List<DiscussionPost>? answers,
    List<DiscussionPost>? comments,
    String? acceptedAnswerId,
    bool unaccept = false,
  }) => Discussion(
    question: discussion.question,
    publicId: discussion.publicId,
    studysetTitle: discussion.studysetTitle,
    location: discussion.location,
    target: discussion.target,
    answers: answers ?? discussion.answers,
    comments: comments ?? discussion.comments,
    acceptedAnswerId: unaccept
        ? null
        : acceptedAnswerId ?? discussion.acceptedAnswerId,
    asker: discussion.asker,
    author: discussion.author,
    refusal: discussion.refusal,
  );
}